The invention relates to a high-pressure discharge lamp provided with a discharge vessel with a ceramic wall which has an outer surface on which a metallic coating is present.
The invention also relates to a method of manufacturing such a lamp.
The term "ceramic wall" in the present description and claims is understood to mean a wall made of either translucent crystalline metal oxide such as, for example, monocrystalline sapphire or, for example, gastight polycrystalline aluminium oxide, or a wall of translucent gastight sintered polycrystalline AlN.
A lamp of the kind mentioned in the opening paragraph is known from EP-A-0002848. To promote lamp ignition, the outer surface of the discharge vessel of the known lamp is provided with an electrically conducting ignition strip in the form of a metallic coating. The strip is adhered to the outer surface of the wall of the discharge vessel in the form of a mixture of metal and metal-oxide particles by means of heating. A metallic coating of a portion of the outer surface of the discharge vessel wall is also known in the form of a heat shield. The aim of this is to exert a positive influence on the heat balance of the lamp. Such a coating is known from inter alia EP-A-034 4 433. The metallic coating may be vapour-deposited in vacuum or provided as a paste which is subsequently cured.
It is found that the metallic coating thus obtained often shows defects during lamp life, in the form of fractures or cracks in the coating or detaching of the coating from the ceramic wall. Such defects in an ignition strip adversely affect the ignition-promoting effect thereof. If the defects are found in a coating serving as a heat shield, they will lead to an undefined change in the heat balance of the lamp. This will generally result in undesirable changes in photometric properties (luminous efficacy, colour temperature, colour rendering) of the lamp.